Node EADDRINUSE "address already in use" for a Test Server in CI - Fix Port Conflicts

EADDRINUSE means a server tried to bind a port that another process already holds. In CI it usually means a previous test server was never closed.

What this error means

A test or integration step throws Error: listen EADDRINUSE: address already in use :::3000. A second listener cannot bind because a prior one is still open.

Error: listen EADDRINUSE: address already in use :::3000
    at Server.setupListenHandle [as _listen2] (node:net:1817:16)
    at listenInCluster (node:net:1865:12) {
  code: 'EADDRINUSE', port: 3000
}

Common causes

A previous test server was not closed

A test suite started a listener and a later suite tries to bind the same fixed port without the first being torn down.

Parallel test workers sharing a hardcoded port

Concurrent workers each bind the same fixed port, so all but the first fail.

How to fix it

Bind to an ephemeral port

1. Listen on port 0 so the OS assigns a free port.
2. Read the assigned port back from the server address for client connections.

const server = app.listen(0);
const { port } = server.address();

Always close servers in teardown

1. Close the listener in an afterAll/afterEach hook.
2. Await the close so the next suite can bind cleanly.

afterAll(() => new Promise((r) => server.close(r)));

How to prevent it

• Use ephemeral ports for test servers, close every listener in teardown, and avoid hardcoded ports shared across parallel workers.

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